c-KIT (also known as KIT, CD117, and stem cell factor receptor) is a 145 kDa transmembrane tyrosine kinase protein that acts as a type-III receptor (Pereira et al. J. Carcin. 2005, 4, pg. 19). The c-KIT proto-oncogene, located on chromosome 4q11-21, encodes the c-KIT receptor, whose ligand is the stem cell factor (SCF, steel factor, kit ligand, mast cell growth factor, Morstyn, G. et al. Oncology 1994, 51 (2), pg. 205; Yarden, Y. et al. Embo. J. 1987, 6 (11), pg. 3341). The receptor has tyrosine-protein kinase activity and binding of the ligand SCF leads to the autophosphorylation of c-KIT and its association with substrates such as phosphatidylinositol 3-kinase (PI3K). Tyrosine phosphorylation by protein tyrosine kinases is of particular importance in cellular signaling and can mediate signals for major cellular processes, such as proliferation, survival, differentiation, apoptosis, attachment, invasiveness and migration. Defects in c-KIT are a cause of piebaldism, an autosomal dominant genetic developmental abnormality of pigmentation characterized by congenital patches of white skin and hair that lack melanocytes. Gain-of-function mutations of the c-KIT gene and the expression of constitutively phosphorylated c-KIT are found in most gastrointestinal stromal tumors GIST) and mastocytosis. Further, almost all gonadal seminomas/dysgerminomas exhibit c-KIT membranous staining, and several reports have clarified that some (10-25%) have a c-KIT gene mutation (Sakuma, Y. et al. Cancer Sci. 2004, 95 (9), pg. 716). c-KIT defects have also been associated with testicular tumors including germ cell tumors (GCT) and testicular germ cell tumors (TGCT).
The role of c-KIT expression has been studied in hematologic and solid tumors, such as acute leukemias (Cortes, J. et al. Cancer 2003, 97 (11), pg. 2760) and GIST (Fletcher, J. et al. Hum. Pathol. 2002, 33 (5), pg. 459). The clinical importance of c-KIT expression in malignant tumors relies on studies with Gleevec® (imatinib mesylate, STI571 (signal transduction inhibitor number 571), Novartis Pharma AG Basel, Switzerland) that specifically inhibits tyrosine kinase receptors (Lefevre, G. et al. J. Biol. Chem. 2004, 279 (30), pg. 31769). Moreover, a clinically relevant breakthrough has been the finding of anti-tumor effects of this compound in GIST, a group of tumors regarded as being generally resistant to conventional chemotherapy (de Silva, C. M.; Reid, R. Pathol. Oncol. Res. 2003, 9 (1), pp. 13-19). Most GISTs have primary activating mutations in the genes encoding the closely related RTKs c-KIT (75-80% of GIST) or PDGFRα (8% of the non-c-KIT mutated GIST). c-KIT and PDGFRα mutations are mutually exclusive in GIST (Rubin et al. Lancet 2007, 369, pg. 1731). The majority of primary GIST-causing c-KIT mutations affect the juxtamembrane (JM) region of the protein encoded by exon 11 (i.e. V560D) and consist of in-frame deletions or insertions, or missense mutations. c-KIT exon 11 mutations have been identified as primary mutations in approximately 75% of GISTs. Such JM domain mutations disrupt the autoinhibition mechanism of c-KIT kinase, leading to constitutive kinase activity and cell-transforming events causative of GIST (Chen, L. L. et al. Clin. Cancer Res. 2005, 11, pg. 3668-3677; Mol, C. D., et al. J. Biol. Chem. 2004, 279, pg. 31655-31663).
GIST most often become Gleevec® resistant, and molecularly targeted small molecule therapies that target c-KIT secondary mutations remain elusive. GIST patients who relapse after treatment with Gleevec® or Sutent® have disease still driven by c-KIT mutations. These secondary mutations occur on the same alleles as the primary JM-region mutation, and thus represent even more aggressive activated forms of c-KIT than the original primary mutation. These secondary mutants of c-KIT identified in GIST lead to acquired drug resistance. Secondary mutations are found in the extracellular domain of c-KIT (exon 9, i.e. AY501-502 duplication/insertion), ATP binding pocket (exon 13, i.e. K642E, V654A; exon 14, i.e. T670I), and activation loop (exon 17, i.e. N822K, D816H, D816V, D820A). These various secondary c-KIT mutations have been reported: Heinrich, M. C. et al. J. Clin. Oncol. 2006, 24, pg. 4764-4774; Debiec-Rychter, M., et al. Gastroenterology 2005, 128, pg. 270-279; Wardelmann, E., et al. Lancet Oncol. 2005, 6, pg. 249-251; Antonescu, C. R., et al. Clin. Cancer. Res. 2005, 11, pg. 4182-4190. Sunitinib malate (Sutent™, Pfizer) is an inhibitor of multiple RTKs, notably in this context, c-KIT and PDGFRα, and has been shown to be effective against certain imatinib-resistant c-KIT mutants, such as the ATP-binding pocket mutants V654A and T670I. Certain Gleevec®-resistant mutants are also resistant to sunitinib, such as D816H and D816V which are located in the activation loop of the c-KIT catalytic domain encoded by exon 17 (Corless et al. J. Clin. Oncol. 2004, 22, pg. 3813; Heinrich et al. J. Clin. Oncol. 2008, 26, pg. 5352; Gajiwala et al. Proc. Natl. Acad. Sci. USA 2009, 106:1542). Median survival after progression due to Gleevec®-resistance remains relatively short.
It has been demonstrated that complex, multiple secondary c-KIT mutations can arise and vary within individual patients, such variation in mutational status of c-KIT being demonstrated by biopsy samples obtained from different progressing metastases within each patient (Wardelmann, E., et al. Lancet Oncol. 2005, 6, pg. 249-251; Fletcher, J. A. and Rubin, B. P., Curr. Opin in Genetics & Develop., 2007, 17, pg. 3-7). This complex c-KIT mutational heterogeneity within individual patients underscores an unmet medical need to identify inhibitors of c-KIT kinase that are effective across a broad spectrum of c-KIT primary and secondary mutations. Such a broad spectrum c-KIT inhibitor would be of high therapeutic value in the treatment of refractory GIST patients.